1. Field of the Invention
The present invention relates to a control device and a control method for a power train, and more particularly to a technology for controlling a power train of a hybrid vehicle including an engine and a rotating electric machine as mechanical power sources.
2. Description of the Background Art
Conventionally, there is known a hybrid vehicle including an internal combustion engine and a rotating electric machine as driving sources. In such a hybrid vehicle, the internal combustion engine and the rotating electric machine are separately used in accordance with a traveling condition of the vehicle. For example, the internal combustion engine is mainly used when the vehicle travels at a high speed, while the rotating electric machine is mainly used when the vehicle travels at an intermediate or low speed. In some of this hybrid vehicle, a differential mechanism in which the engine and the rotating electric machine are coupled with each other functions as a continuously variable transmission.
In particular, Japanese Patent Laying-Open No. 2006-094617 discloses a vehicle driving device adapted to transmit an output of a driving power source to drive wheels via a differential unit including the above described differential mechanism and a stepwise shift transmission coupled with a rear stage of the differential unit by a transmission member.
A control device of the vehicle driving device disclosed in Japanese Patent Laying-Open No. 2006-094617 includes a switching clutch or a switching brake for switching the transmission mechanism between a continuous shift state and a stepwise shift state, so that a driving device can advantageously obtain both of a fuel consumption improving effect of a transmission device in which a gear ratio is electrically converted and a high transmitting efficiency of a stepwise shift transmission device in which power is mechanically transmitted.
However, in the vehicle driving device disclosed in Japanese Patent Laying-Open No. 2006-094617, the transmission member which couples the differential mechanism with the stepwise shift transmission is free with no engagement in the stepwise shift transmission when a parking (P) position or a neutral (N) position is selected. Therefore, the engine is idly operated in a state where one of rotating elements (ring gear) constituting the differential mechanism is free with no engagement, and a behavior of an engine rotation speed at the time of the idle speed control (ISC) possibly becomes unstable due to the differential operation in the differential mechanism. The term “rotation speed” recited in the present specification denotes the number of rotations per unit time (typically, rpm) unless any specific description is given.
When a learning control that corrects an amount set by the ISC based on an engine rotation speed actual value during a period when the ISC is implemented in the foregoing condition, the rotation speed may in an idle state be excessively increased or significantly reduced to unfavorably invite an engine stall when the ISC is implemented next due to misleading learning.